The actions of the adipocyte-derived hormone leptin on lipid metabolism are proposed to be important in preventing the development of tissue lipotoxicity and lipid-related insulin resistance. This is best illustrated in leptin-deficient states, where chronic administration of exogenous leptin corrects systemic dyslipidemia, excessive storage of lipid in peripheral tissues such as the liver, and insulin resistance associated with this condition. Furthermore, states of leptin resistance such as human obesity and diet-induced obesity (DIO) are characterized by a similar metabolic phenotype to leptin deficient conditions, implicating a role for a loss of leptin action in the pathogenesis of the metabolic abnormalities of obesity. However, despite some progress, our understanding of the mechanisms of leptin regulation of lipid metabolism and insulin action, and the mechanisms of leptin resistance remains poor. Our work over the last decade has focused on the effects and mechanisms of leptin action on lipid metabolism and insulin sensitivity, with our recent work focusing on liver, a central organ in the regulation of whole-body lipid homeostasis and insulin sensitivity. In brief these studies established a novel role for leptin in the acute regulation of hepatic oxidative and VLDL metabolism, effects that are impaired in obesity, and identified the biochemical basis for these effects. In preliminary data, we present evidence that many of these effects are mediated by specialized liver macrophages (Kupffer cells). These data demonstrate a previously unappreciated role for myeloid cells in mediating the metabolic actions of leptin. The central objective of the current proposal is to extend these observations to address (i) the role of macrophages in mediating the metabolic actions of leptin, and (ii) the biochemical and molecular mechanisms of macrophage leptin action. In undertaking these studies, we will increase our understanding of the mechanisms of leptin action and the contribution of leptin resistance to the metabolic abnormalities of obesity. To address these questions we will utilize a range of models, including genetically manipulated mice and cell based co-culture systems, and approaches, including metabolic, biochemical, and molecular analysis.